The techniques for the transmission/reception of digital signals in systems comprising multiple antennas have many advantages, especially for mobile access networks. Indeed, such techniques can be used to increase a transmission bit rate, capacity or robustness for these multi-antenna systems without in any way requiring an increase in transmission power or allocated frequency bandwidth.
There also exists a known technique in the prior art used to attain open-loop capacity for frequency non-selective fading channels, assuming that there are independent Gaussian inputs. This technique relies on the implementation of an MMSE (minimum mean-square error) type receiver combined with successive interference cancellation and on the implementation of a control of rates per antennas, known as PARC (Per Antenna Rate Control) enabling the regular matching of the rate of each transmitting antenna to the capacity of the corresponding equivalent transmission channel, thus preventing a propagation of errors at the level of the interference cancellation.
It may recalled that the term “rate” conventionally refers to a choice of a modulation and encoding scheme (MCS), i.e. it comprises:                The channel code rate: for example ¼, ⅓, ½ etc;        the order of the modulation chosen: for example BPSK (Binary Phase Shift Keying), QPSK (Quadrature PSK), 16 QAM (16-Quadrature Amplitude Modulation) etc.        
In particular, the technique referenced SIC-PARC (Successive Interference Cancellation—PARC) enables the transmission of independent data streams which may have different rates on each of the transmit antennas of a multi-antenna system. In this technique, the power given at transmission may be uniformly distributed among all the transmit antennas.
At reception, the Signal-to-Interference-plus-Noise Ratio (SINR) associated with each of the transmit antennas is determined from an estimation of the transmission channel.
Thus, a receiver knowing a family of modulation and encoding schemes available at transmission can determine the modulation and encoding techniques to be used in transmission to minimize the difference with the theoretical performances and forward these elements to the transmitter by means of a piece of information coming from the receiver, also called “instantaneous partial feedback” carried for example by a CQI (Channel Quality Indicator) message.
More specifically again, the transmission standard considered is used to define a family of modulation and encoding schemes and therefore discrete rates enabling a receiver to determine the MCS and hence the discrete per antenna rates to be used.
An improvement in this prior-art technique, known as S-PARC, is described especially in Ericsson, “Selective Per Antenna Rate Control”, 3GPP TSG RAN WG1, February 2004, illustrated with reference to FIG. 6. This technique has the advantage of enabling the selection of a set of active transmit antennas from among the available transmit antennas. Thus, two sub-sets of transmit antennas are distinguished: active antennas and non-active antennas.
Thus, rather than all the streams associated with each transmit antenna independently, only the streams associated with an active transmit antenna are considered. Thus, the number of independent streams transmitted, equal to the number of active antennas, is smaller than or equal to the total number of transmit antennas.
The transmit antennas can be selected as a function of the quality of the transmission channel and/or the correlation of the transmit antennas, so as to maximize especially the sum of the rates on all the streams and hence the total capacity of the system.
In other words, the allocation of discrete rates to each transmit antenna is based on the reception of one CQI per antenna, indicating the discrete per antenna rate (MCS) to be used.
Thus, as illustrated in FIG. 6, a receiver starts by estimating the transmission channel during a first step 61. In a following step 62, the receiver determines the SINR ratio associated with each of the transmit antennas and sends this information back to the transmitter (as feedback). The associated transmitter can then select (63) the active transmit antennas and deduce therefrom the theoretical rate to be used per antenna and the modulation and coding scheme MCS to be used (64) to minimize the difference with the theoretical rate. Finally, the transmitter transmits data packets on the basis of the active transmit antennas during a step 65.
However, one drawback of this technique is that it does not take account of the set of discrete rates available at transmission during the selection of the active antennas. In practice, this technique therefore suffers from a quantification noise due to the family of discrete rates available. Consequently, the discretization of the theoretical rates applied subsequently (i.e. the choice of modulation and coding scheme MCS to be used to minimize the difference with the theoretical rate, among the available MCS values) prompts a loss of spectral efficiency.
Another technique seeks to optimize the power values and the rates allocated. For example, S. T. Chung et al in “Approaching the MIMO Capacity with low-rate feed-back channel in V-BLAST” (Eurasip Journal on applied signal processing, pages 762-771, 2004), propose a technique for the joint optimizing of the decoding order and the power values allocated to each of the transmit antennas so as to comply with the transmission constraints, especially the constraints related to discrete rates (MCS). This technique is known as the “SRPQ2: efficient decoding order” (SRPQ2=successive rate and power quantization).
This technique also uses a scalar type encoding which is independent for each antenna, chosen so as to approximate a maximum capacity defined by Shannon's limit, especially in order to obtain a low binary error rate and thus maximize the total capacity of the system.
However, one drawback of this technique of joint optimization of rates and power values associated with the search for optimum decoding order to match the discrete distribution of rates available is that it works only in the context of scalar encoding.
It is observed indeed that the prior-art techniques continue to process each transmit antenna separately (scalar encoding) or else all the transmit antennas simultaneously (multidimensional encoding).